The invention relates to a method for monitoring an exhaust gas purifying component with gas storage capability for purifying exhaust gases from an internal combustion engine.
International Patent Document WO 96/01364 discloses a catalyst monitoring system for monitoring an exhaust gas purifying component with gas storage capability for purifying exhaust gases from an internal combustion engine. The system comprises a thin-film resistance thermometer as a temperature sensor, which may be incorporated into an exhaust gas purifying component. The temperature sensor detects temperature changes occurring at a catalyst material during oxidation of reducing exhaust gas constituents as a result of the heat of reaction then released. These changes serve as a measure for assessing the activity of the catalyst with regard to the relevant oxidation reactions. (Impaired catalytic activity results in reduced conversion of the oxidizable exhaust gas constituents and thus smaller temperature changes.)
Comparison with reference values allows catalyst impairment to be determined. However, the magnitude of the temperature change occurring upon conversion of the oxidizable constituents is heavily dependent on parameters such as exhaust gas composition, flow rate, temperature, and the like. An evaluation of the temperature changes with regard to catalyst activity is therefore difficult, and may result in misinterpretations.
It is therefore an object of the invention to provide a method which enables improved monitoring of an exhaust gas purifying component with gas storage capability.
This and other objects and advantages are achieved by the method according to the invention for monitoring an exhaust gas purifying component with gas storage capability, in which a temperature change of the exhaust gas purifying component and/or of the gas storage material is evaluated based substantially on the heat tonality of the modification transition of the gas storage material. Storage of a gas in the gas storage material is associated with a modification transition of the gas storage material, which is accompanied by a heat tonality. The modification transition preferably involves a change in the chemical structure of the gas storage material as a result of gas storage by a chemical reaction of the gas storage material with the gas. Examples here are storage of nitrogen oxides in the form of nitrates in a nitrogen oxide storage material, which is present in the initial state for example as carbonate. A chemical structural change also takes place upon storage of oxygen in an oxygen storage material such as cerium oxide or a cerium oxide-containing mixed oxide, in which case the material develops into an oxygen-rich modification of high chemical valency upon oxygen absorption.
However, it may also involve a change in the modification in the form of formation of a chemisorptive bond of the gas storage material with the gas, in which the chemical structure of the gas storage material remains substantially unchanged but the stored gas is bonded to the gas storage material by weak chemical bonds. An example of this is the adsorption of hydrocarbons in storage materials suitable therefor, such as for example zeolites.
The reaction and/or sorption enthalpies associated with these chemical reactions or bonding processes lead to a so-called heat tonality, which in turn has as a consequence a measurable temperature change of the gas storage material or the exhaust gas purifying component. Transition of the gas storage material from a first to a second modification may also involve a combination of the stated modification transitions. Accordingly, the exhaust gas purifying component preferably comprises a catalytic converter and/or an adsorber with a gas storage material, which may bond an exhaust gas component such as oxygen, nitrogen oxide, sulfur oxide, ammonia, hydrocarbon, carbon monoxide, carbon dioxide or water. The method is particularly suitable for monitoring catalytic exhaust gas purifying components of the supported catalyst type. In these components, the gas storage material is applied to a ceramic or metal foil support, preferably in the form of a honeycomb structure.
The extent of the temperature change depends on the one hand on the type of modification transition. By detecting and evaluating the temperature change associated with a modification transition, it is possible to draw conclusions about the type and intensity thereof and to monitor the exhaust gas purifying component accordingly.
On the other hand, the amount of gas storage material affected by the modification transition influences the extent of the temperature change. By evaluating the temperature change associated with the modification transition, it is therefore possible to draw conclusions about the amount of active gas storage material, and to monitor the exhaust gas purifying component for a decrease in material or activity.
As a development of the method, the temperature change is evaluated with regard to aging of the exhaust gas purifying component. For example, as a result of thermal loading, exhaust gas purifying components generally lose their gas storage capability over the course of their service life. Often this takes the form of a loss in activity of the gas storage material as a result of sintering and/or phase transition or phase segregation processes. By evaluating the temperature changes caused by the change in modification of the gas storage material, the extent of aging of the exhaust gas purifying component (and for example an aging index) may therefore be established.
According to a further feature of the invention, the temperature change arising when the operating mode of the internal combustion engine changes is evaluated, preferably, a change in which the composition of the internal combustion engine exhaust gas changes from hypostoichiometric (i.e., reducing), to hyperstoichiometric (i.e., oxidizing). Such a change in operating mode involves a change in the exhaust gas composition (essential exhaust gas constituents). This also affects the storage of exhaust gas constituents in the exhaust gas purifying component through which exhaust gas flows. Consequently, when a change of the stated type takes place in the operating mode, monitoring and assessment of the exhaust gas purifying component may be effectively performed. It is particularly advantageous in this connection, as a further development of the method, to evaluate the increase in temperature that occurs upon a change from traction mode operation of the internal combustion engine with a hypostoichiometric air/fuel ratio (λ<1.0) to overrun mode with a hyperstoichiometric air/fuel ratio (λ>1.0).
According to a further feature of the invention, an exhaust gas purifying component with oxygen storage capability is monitored. This is preferably an oxidation catalyst and/or a three-way catalyst or a nitrogen oxide storage catalyst. With these catalyst types, the oxygen storage capability is important for catalyst function. Using the method according to the invention, it is therefore possible to perform a diagnosis with regard to catalyst efficiency, in particular in the case of the stated catalyst types.
According to a further feature of the invention, an exhaust gas purifying component with a cerium oxide-containing gas storage material is monitored. Cerium oxide-containing materials have an oxygen storage capability, wherein, when oxygen is stored, the proportion of trivalent cerium oxide (Ce2O3) decreases in favor of tetravalent cerium oxide (CeO2). This process is associated with heat release, so that cerium oxide-containing exhaust gas purifying components may be monitored particularly effectively in accordance with the invention.
According to a further feature of the invention, detection of the temperature changes takes place at least two different points in the exhaust gas purifying component. Since exhaust gas purifying components are generally exposed to a decreasing load in the axial direction (i.e., in the direction of exhaust gas through-flow), it is particularly advantageous for temperature changes to be detected at points that are offset axially relative to one another. In this way, monitoring is particularly accurate, and aging can be determined in a spatially resolved manner.
According to a further feature of the invention, detection of the temperature change takes place at a point that is spaced from an exhaust gas inlet side of the exhaust gas purifying component in the direction of exhaust gas flow. Detecting the temperature at a distance from the gas inlet side prevents exothermic oxidation of exhaust gas components occurring principally on the exhaust gas inlet side from disturbing or distorting the heat tonality of a modification transformation of the gas storage material. An especially exothermic modification transition may therefore be particularly reliably detected at a point spaced from the exhaust gas inlet side, further improving monitoring of the exhaust gas purifying component.
In particular it is advantageous if, according to a further aspect of the invention, detection of the temperature increase takes place in an area of the exhaust gas purifying component which is spaced by more than 10 mm, in particular by more than 30 mm from the gas inlet side thereof in the direction of exhaust gas flow. This applies in particular to the exhaust gas purifying component first exposed to the engine's exhaust gas. If one or more in particular catalytic exhaust gas purifying components are arranged upstream of the exhaust gas purifying component, detection of the temperature increase may also take place directly at the inlet-side end of the exhaust gas purifying component.
According to a further feature of the invention, detection of the temperature increase takes place using a temperature sensor which is inserted into the exhaust gas purifying component in such a way that a temperature-sensitive part of the temperature sensor is in heat transfer contact with the gas storage material. In this way, a change in the temperature of the gas storage material resulting from a modification transition may be detected particularly sensitively and accurately. In this respect, it is particularly advantageous if, according to a further aspect of the invention, a temperature sensor with a low thermal mass is used to detect, the temperature increase. This is the case, for example, when the thermal capacity of the relevant temperature sensor part is roughly of the order of magnitude of the gas storage material quantity detected thereby.
Advantageous embodiments of the invention are illustrated in the drawings and are described below. The above-mentioned features and those still to be explained below may in this regard be used not only in the respectively stated combinations of features but also in other combinations or alone, without going beyond the scope of the present invention.